The purpose of this application is to investigate a novel mechanism regulating cellular responses to ionizing radiation (IR) in oral mucosa and to develop effective therapies for treating radiation damage in oral cavity. Exposure to ionizing radiation causes damage to oral mucosa that culminates in oral mucositis, a painful and debilitating disorder highly prevalent in patients receiving radiation therapy for head and neck cancer. IR triggers the onset of radiation-induced premature senescence (RIPS) that mediates tissue damage. Senescent cells secrete growth factors, proteases, and pro-inflammatory cytokines, through a process called senescence- associated secretory phenotype (SASP), which may contribute to the inflammatory phase of oral mucositis in irradiated tissues. At the mechanistic level, we recently found that IR exposure induces premature senescence in normal human oral keratinocytes (NHOK) through epigenetic mechanism that involves activation of histone demethylase, e.g., Jmjd3, loss of polycomb group (PcG) proteins, e.g., Bmi-1 and EZH2, and altered level of trimethylated histone H3K27 (H3K27me3), indicating that normal cell radiation response is in part determined through the balance between Jmjd3 and PcG proteins. When we perturbed the balance between Jmjd3 and PcG by overexpression of Bmi-1, we found that RIPS was strongly inhibited in cells and the global level of H3K27me3 was maintained even after IR exposure. Such radioprotective effects of Bmi-1 overexpression were observed in monolayer culture of NHK, as well as in organotypic raft culture in 3D. Taken together, these results raise a hypothesis that epigenetic changes by the interplay between Jmjd3 and the PcG proteins mediate the onset of RIPS and SASP upon radiation exposure. This hypothesis will be tested by investigating the roles of Jmjd3 and PcG proteins in epigenetic regulation of RIPS and SASP in NHOK in response to IR; determining the occurrence of RIPS and the corresponding epigenetic changes in oral mucosa after exposure to IR in vivo; and determining the effects of altered Jmjd3 or PcG status on RIPS in oral mucosa exposed to radiation. These experiments will elucidate a novel epigenetic mechanism underlying radiation- induced oral mucositis, and lay the foundation for developing novel therapeutics for oral mucositis in future research.